1 /*
2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation. Oracle designates this
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9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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24 */
25
26 package java.util;
27
28 import java.util.function.Predicate;
29 import java.util.stream.Stream;
30 import java.util.stream.StreamSupport;
31
32 /**
33 * The root interface in the <i>collection hierarchy</i>. A collection
34 * represents a group of objects, known as its <i>elements</i>. Some
35 * collections allow duplicate elements and others do not. Some are ordered
36 * and others unordered. The JDK does not provide any <i>direct</i>
37 * implementations of this interface: it provides implementations of more
38 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
39 * is typically used to pass collections around and manipulate them where
40 * maximum generality is desired.
41 *
42 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
43 * duplicate elements) should implement this interface directly.
44 *
45 * <p>All general-purpose <tt>Collection</tt> implementation classes (which
46 * typically implement <tt>Collection</tt> indirectly through one of its
47 * subinterfaces) should provide two "standard" constructors: a void (no
48 * arguments) constructor, which creates an empty collection, and a
49 * constructor with a single argument of type <tt>Collection</tt>, which
50 * creates a new collection with the same elements as its argument. In
51 * effect, the latter constructor allows the user to copy any collection,
52 * producing an equivalent collection of the desired implementation type.
53 * There is no way to enforce this convention (as interfaces cannot contain
54 * constructors) but all of the general-purpose <tt>Collection</tt>
55 * implementations in the Java platform libraries comply.
56 *
57 * <p>The "destructive" methods contained in this interface, that is, the
58 * methods that modify the collection on which they operate, are specified to
59 * throw <tt>UnsupportedOperationException</tt> if this collection does not
60 * support the operation. If this is the case, these methods may, but are not
61 * required to, throw an <tt>UnsupportedOperationException</tt> if the
62 * invocation would have no effect on the collection. For example, invoking
63 * the {@link #addAll(Collection)} method on an unmodifiable collection may,
64 * but is not required to, throw the exception if the collection to be added
65 * is empty.
66 *
67 * <p><a name="optional-restrictions">
68 * Some collection implementations have restrictions on the elements that
69 * they may contain.</a> For example, some implementations prohibit null elements,
70 * and some have restrictions on the types of their elements. Attempting to
71 * add an ineligible element throws an unchecked exception, typically
72 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting
73 * to query the presence of an ineligible element may throw an exception,
74 * or it may simply return false; some implementations will exhibit the former
75 * behavior and some will exhibit the latter. More generally, attempting an
76 * operation on an ineligible element whose completion would not result in
77 * the insertion of an ineligible element into the collection may throw an
78 * exception or it may succeed, at the option of the implementation.
79 * Such exceptions are marked as "optional" in the specification for this
80 * interface.
81 *
82 * <p>It is up to each collection to determine its own synchronization
83 * policy. In the absence of a stronger guarantee by the
84 * implementation, undefined behavior may result from the invocation
85 * of any method on a collection that is being mutated by another
86 * thread; this includes direct invocations, passing the collection to
87 * a method that might perform invocations, and using an existing
88 * iterator to examine the collection.
89 *
90 * <p>Many methods in Collections Framework interfaces are defined in
91 * terms of the {@link Object#equals(Object) equals} method. For example,
92 * the specification for the {@link #contains(Object) contains(Object o)}
93 * method says: "returns <tt>true</tt> if and only if this collection
94 * contains at least one element <tt>e</tt> such that
95 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should
96 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
97 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
98 * invoked for any element <tt>e</tt>. Implementations are free to implement
99 * optimizations whereby the <tt>equals</tt> invocation is avoided, for
100 * example, by first comparing the hash codes of the two elements. (The
101 * {@link Object#hashCode()} specification guarantees that two objects with
102 * unequal hash codes cannot be equal.) More generally, implementations of
103 * the various Collections Framework interfaces are free to take advantage of
104 * the specified behavior of underlying {@link Object} methods wherever the
105 * implementor deems it appropriate.
106 *
107 * <p>Some collection operations which perform recursive traversal of the
108 * collection may fail with an exception for self-referential instances where
109 * the collection directly or indirectly contains itself. This includes the
110 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
111 * methods. Implementations may optionally handle the self-referential scenario,
112 * however most current implementations do not do so.
113 *
114 * <p>This interface is a member of the
115 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
116 * Java Collections Framework</a>.
117 *
118 * @implSpec
119 * The default method implementations (inherited or otherwise) do not apply any
120 * synchronization protocol. If a {@code Collection} implementation has a
121 * specific synchronization protocol, then it must override default
122 * implementations to apply that protocol.
123 *
124 * @param <E> the type of elements in this collection
125 *
126 * @author Josh Bloch
127 * @author Neal Gafter
128 * @see Set
129 * @see List
130 * @see Map
131 * @see SortedSet
132 * @see SortedMap
133 * @see HashSet
134 * @see TreeSet
135 * @see ArrayList
136 * @see LinkedList
137 * @see Vector
138 * @see Collections
139 * @see Arrays
140 * @see AbstractCollection
141 * @since 1.2
142 */
143
144 public interface Collection<E> extends Iterable<E> {
145 // Query Operations
146
147 /**
148 * Returns the number of elements in this collection. If this collection
149 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
150 * <tt>Integer.MAX_VALUE</tt>.
151 *
152 * @return the number of elements in this collection
153 */
154 int size();
155
156 /**
157 * Returns <tt>true</tt> if this collection contains no elements.
158 *
159 * @return <tt>true</tt> if this collection contains no elements
160 */
161 boolean isEmpty();
162
163 /**
164 * Returns <tt>true</tt> if this collection contains the specified element.
165 * More formally, returns <tt>true</tt> if and only if this collection
166 * contains at least one element <tt>e</tt> such that
167 * <tt>(o==null ? e==null : o.equals(e))</tt>.
168 *
169 * @param o element whose presence in this collection is to be tested
170 * @return <tt>true</tt> if this collection contains the specified
171 * element
172 * @throws ClassCastException if the type of the specified element
173 * is incompatible with this collection
174 * (<a href="#optional-restrictions">optional</a>)
175 * @throws NullPointerException if the specified element is null and this
176 * collection does not permit null elements
177 * (<a href="#optional-restrictions">optional</a>)
178 */
179 boolean contains(Object o);
180
181 /**
182 * Returns an iterator over the elements in this collection. There are no
183 * guarantees concerning the order in which the elements are returned
184 * (unless this collection is an instance of some class that provides a
185 * guarantee).
186 *
187 * @return an <tt>Iterator</tt> over the elements in this collection
188 */
189 Iterator<E> iterator();
190
191 /**
192 * Returns an array containing all of the elements in this collection.
193 * If this collection makes any guarantees as to what order its elements
194 * are returned by its iterator, this method must return the elements in
195 * the same order.
196 *
197 * <p>The returned array will be "safe" in that no references to it are
198 * maintained by this collection. (In other words, this method must
199 * allocate a new array even if this collection is backed by an array).
200 * The caller is thus free to modify the returned array.
201 *
202 * <p>This method acts as bridge between array-based and collection-based
203 * APIs.
204 *
205 * @return an array containing all of the elements in this collection
206 */
207 Object[] toArray();
208
209 /**
210 * Returns an array containing all of the elements in this collection;
211 * the runtime type of the returned array is that of the specified array.
212 * If the collection fits in the specified array, it is returned therein.
213 * Otherwise, a new array is allocated with the runtime type of the
214 * specified array and the size of this collection.
215 *
216 * <p>If this collection fits in the specified array with room to spare
217 * (i.e., the array has more elements than this collection), the element
218 * in the array immediately following the end of the collection is set to
219 * <tt>null</tt>. (This is useful in determining the length of this
220 * collection <i>only</i> if the caller knows that this collection does
221 * not contain any <tt>null</tt> elements.)
222 *
223 * <p>If this collection makes any guarantees as to what order its elements
224 * are returned by its iterator, this method must return the elements in
225 * the same order.
226 *
227 * <p>Like the {@link #toArray()} method, this method acts as bridge between
228 * array-based and collection-based APIs. Further, this method allows
229 * precise control over the runtime type of the output array, and may,
230 * under certain circumstances, be used to save allocation costs.
231 *
232 * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
233 * The following code can be used to dump the collection into a newly
234 * allocated array of <tt>String</tt>:
235 *
236 * <pre>
237 * String[] y = x.toArray(new String[0]);</pre>
238 *
239 * Note that <tt>toArray(new Object[0])</tt> is identical in function to
240 * <tt>toArray()</tt>.
241 *
242 * @param <T> the runtime type of the array to contain the collection
243 * @param a the array into which the elements of this collection are to be
244 * stored, if it is big enough; otherwise, a new array of the same
245 * runtime type is allocated for this purpose.
246 * @return an array containing all of the elements in this collection
247 * @throws ArrayStoreException if the runtime type of the specified array
248 * is not a supertype of the runtime type of every element in
249 * this collection
250 * @throws NullPointerException if the specified array is null
251 */
252 <T> T[] toArray(T[] a);
253
254 // Modification Operations
255
256 /**
257 * Ensures that this collection contains the specified element (optional
258 * operation). Returns <tt>true</tt> if this collection changed as a
259 * result of the call. (Returns <tt>false</tt> if this collection does
260 * not permit duplicates and already contains the specified element.)<p>
261 *
262 * Collections that support this operation may place limitations on what
263 * elements may be added to this collection. In particular, some
264 * collections will refuse to add <tt>null</tt> elements, and others will
265 * impose restrictions on the type of elements that may be added.
266 * Collection classes should clearly specify in their documentation any
267 * restrictions on what elements may be added.<p>
268 *
269 * If a collection refuses to add a particular element for any reason
270 * other than that it already contains the element, it <i>must</i> throw
271 * an exception (rather than returning <tt>false</tt>). This preserves
272 * the invariant that a collection always contains the specified element
273 * after this call returns.
274 *
275 * @param e element whose presence in this collection is to be ensured
276 * @return <tt>true</tt> if this collection changed as a result of the
277 * call
278 * @throws UnsupportedOperationException if the <tt>add</tt> operation
279 * is not supported by this collection
280 * @throws ClassCastException if the class of the specified element
281 * prevents it from being added to this collection
282 * @throws NullPointerException if the specified element is null and this
283 * collection does not permit null elements
284 * @throws IllegalArgumentException if some property of the element
285 * prevents it from being added to this collection
286 * @throws IllegalStateException if the element cannot be added at this
287 * time due to insertion restrictions
288 */
289 boolean add(E e);
290
291 /**
292 * Removes a single instance of the specified element from this
293 * collection, if it is present (optional operation). More formally,
294 * removes an element <tt>e</tt> such that
295 * <tt>(o==null ? e==null : o.equals(e))</tt>, if
296 * this collection contains one or more such elements. Returns
297 * <tt>true</tt> if this collection contained the specified element (or
298 * equivalently, if this collection changed as a result of the call).
299 *
300 * @param o element to be removed from this collection, if present
301 * @return <tt>true</tt> if an element was removed as a result of this call
302 * @throws ClassCastException if the type of the specified element
303 * is incompatible with this collection
304 * (<a href="#optional-restrictions">optional</a>)
305 * @throws NullPointerException if the specified element is null and this
306 * collection does not permit null elements
307 * (<a href="#optional-restrictions">optional</a>)
308 * @throws UnsupportedOperationException if the <tt>remove</tt> operation
309 * is not supported by this collection
310 */
311 boolean remove(Object o);
312
313
314 // Bulk Operations
315
316 /**
317 * Returns <tt>true</tt> if this collection contains all of the elements
318 * in the specified collection.
319 *
320 * @param c collection to be checked for containment in this collection
321 * @return <tt>true</tt> if this collection contains all of the elements
322 * in the specified collection
323 * @throws ClassCastException if the types of one or more elements
324 * in the specified collection are incompatible with this
325 * collection
326 * (<a href="#optional-restrictions">optional</a>)
327 * @throws NullPointerException if the specified collection contains one
328 * or more null elements and this collection does not permit null
329 * elements
330 * (<a href="#optional-restrictions">optional</a>),
331 * or if the specified collection is null.
332 * @see #contains(Object)
333 */
334 boolean containsAll(Collection<?> c);
335
336 /**
337 * Adds all of the elements in the specified collection to this collection
338 * (optional operation). The behavior of this operation is undefined if
339 * the specified collection is modified while the operation is in progress.
340 * (This implies that the behavior of this call is undefined if the
341 * specified collection is this collection, and this collection is
342 * nonempty.)
343 *
344 * @param c collection containing elements to be added to this collection
345 * @return <tt>true</tt> if this collection changed as a result of the call
346 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
347 * is not supported by this collection
348 * @throws ClassCastException if the class of an element of the specified
349 * collection prevents it from being added to this collection
350 * @throws NullPointerException if the specified collection contains a
351 * null element and this collection does not permit null elements,
352 * or if the specified collection is null
353 * @throws IllegalArgumentException if some property of an element of the
354 * specified collection prevents it from being added to this
355 * collection
356 * @throws IllegalStateException if not all the elements can be added at
357 * this time due to insertion restrictions
358 * @see #add(Object)
359 */
360 boolean addAll(Collection<? extends E> c);
361
362 /**
363 * Removes all of this collection's elements that are also contained in the
364 * specified collection (optional operation). After this call returns,
365 * this collection will contain no elements in common with the specified
366 * collection.
367 *
368 * @param c collection containing elements to be removed from this collection
369 * @return <tt>true</tt> if this collection changed as a result of the
370 * call
371 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
372 * is not supported by this collection
373 * @throws ClassCastException if the types of one or more elements
374 * in this collection are incompatible with the specified
375 * collection
376 * (<a href="#optional-restrictions">optional</a>)
377 * @throws NullPointerException if this collection contains one or more
378 * null elements and the specified collection does not support
379 * null elements
380 * (<a href="#optional-restrictions">optional</a>),
381 * or if the specified collection is null
382 * @see #remove(Object)
383 * @see #contains(Object)
384 */
385 boolean removeAll(Collection<?> c);
386
387 /**
388 * Removes all of the elements of this collection that satisfy the given
389 * predicate. Errors or runtime exceptions thrown during iteration or by
390 * the predicate are relayed to the caller.
391 *
392 * @implSpec
393 * The default implementation traverses all elements of the collection using
394 * its {@link #iterator}. Each matching element is removed using
395 * {@link Iterator#remove()}. If the collection's iterator does not
396 * support removal then an {@code UnsupportedOperationException} will be
397 * thrown on the first matching element.
398 *
399 * @param filter a predicate which returns {@code true} for elements to be
400 * removed
401 * @return {@code true} if any elements were removed
402 * @throws NullPointerException if the specified filter is null
403 * @throws UnsupportedOperationException if elements cannot be removed
404 * from this collection. Implementations may throw this exception if a
405 * matching element cannot be removed or if, in general, removal is not
406 * supported.
407 * @since 1.8
408 */
409 default boolean removeIf(Predicate<? super E> filter) {
410 Objects.requireNonNull(filter);
411 boolean removed = false;
412 final Iterator<E> each = iterator();
413 while (each.hasNext()) {
414 if (filter.test(each.next())) {
415 each.remove();
416 removed = true;
417 }
418 }
419 return removed;
420 }
421
422 /**
423 * Retains only the elements in this collection that are contained in the
424 * specified collection (optional operation). In other words, removes from
425 * this collection all of its elements that are not contained in the
426 * specified collection.
427 *
428 * @param c collection containing elements to be retained in this collection
429 * @return <tt>true</tt> if this collection changed as a result of the call
430 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
431 * is not supported by this collection
432 * @throws ClassCastException if the types of one or more elements
433 * in this collection are incompatible with the specified
434 * collection
435 * (<a href="#optional-restrictions">optional</a>)
436 * @throws NullPointerException if this collection contains one or more
437 * null elements and the specified collection does not permit null
438 * elements
439 * (<a href="#optional-restrictions">optional</a>),
440 * or if the specified collection is null
441 * @see #remove(Object)
442 * @see #contains(Object)
443 */
444 boolean retainAll(Collection<?> c);
445
446 /**
447 * Removes all of the elements from this collection (optional operation).
448 * The collection will be empty after this method returns.
449 *
450 * @throws UnsupportedOperationException if the <tt>clear</tt> operation
451 * is not supported by this collection
452 */
453 void clear();
454
455
456 // Comparison and hashing
457
458 /**
459 * Compares the specified object with this collection for equality. <p>
460 *
461 * While the <tt>Collection</tt> interface adds no stipulations to the
462 * general contract for the <tt>Object.equals</tt>, programmers who
463 * implement the <tt>Collection</tt> interface "directly" (in other words,
464 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
465 * or a <tt>List</tt>) must exercise care if they choose to override the
466 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
467 * course of action is to rely on <tt>Object</tt>'s implementation, but
468 * the implementor may wish to implement a "value comparison" in place of
469 * the default "reference comparison." (The <tt>List</tt> and
470 * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
471 *
472 * The general contract for the <tt>Object.equals</tt> method states that
473 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
474 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
475 * and <tt>Set.equals</tt> state that lists are only equal to other lists,
476 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a
477 * collection class that implements neither the <tt>List</tt> nor
478 * <tt>Set</tt> interface must return <tt>false</tt> when this collection
479 * is compared to any list or set. (By the same logic, it is not possible
480 * to write a class that correctly implements both the <tt>Set</tt> and
481 * <tt>List</tt> interfaces.)
482 *
483 * @param o object to be compared for equality with this collection
484 * @return <tt>true</tt> if the specified object is equal to this
485 * collection
486 *
487 * @see Object#equals(Object)
488 * @see Set#equals(Object)
489 * @see List#equals(Object)
490 */
491 boolean equals(Object o);
492
493 /**
494 * Returns the hash code value for this collection. While the
495 * <tt>Collection</tt> interface adds no stipulations to the general
496 * contract for the <tt>Object.hashCode</tt> method, programmers should
497 * take note that any class that overrides the <tt>Object.equals</tt>
498 * method must also override the <tt>Object.hashCode</tt> method in order
499 * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
500 * In particular, <tt>c1.equals(c2)</tt> implies that
501 * <tt>c1.hashCode()==c2.hashCode()</tt>.
502 *
503 * @return the hash code value for this collection
504 *
505 * @see Object#hashCode()
506 * @see Object#equals(Object)
507 */
508 int hashCode();
509
510 /**
511 * Creates a {@link Spliterator} over the elements in this collection.
512 *
513 * Implementations should document characteristic values reported by the
514 * spliterator. Such characteristic values are not required to be reported
515 * if the spliterator reports {@link Spliterator#SIZED} and this collection
516 * contains no elements.
517 *
518 * <p>The default implementation should be overridden by subclasses that
519 * can return a more efficient spliterator. In order to
520 * preserve expected laziness behavior for the {@link #stream()} and
521 * {@link #parallelStream()} methods, spliterators should either have the
522 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
523 * <em><a href="Spliterator.html#binding">late-binding</a></em>.
524 * If none of these is practical, the overriding class should describe the
525 * spliterator's documented policy of binding and structural interference,
526 * and should override the {@link #stream()} and {@link #parallelStream()}
527 * methods to create streams using a {@code Supplier} of the spliterator,
528 * as in:
529 * <pre>{@code
530 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
531 * }</pre>
532 * <p>These requirements ensure that streams produced by the
533 * {@link #stream()} and {@link #parallelStream()} methods will reflect the
534 * contents of the collection as of initiation of the terminal stream
535 * operation.
536 *
537 * @implSpec
538 * The default implementation creates a
539 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
540 * from the collections's {@code Iterator}. The spliterator inherits the
541 * <em>fail-fast</em> properties of the collection's iterator.
542 * <p>
543 * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
544 *
545 * @implNote
546 * The created {@code Spliterator} additionally reports
547 * {@link Spliterator#SUBSIZED}.
548 *
549 * <p>If a spliterator covers no elements then the reporting of additional
550 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
551 * does not aid clients to control, specialize or simplify computation.
552 * However, this does enable shared use of an immutable and empty
553 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
554 * empty collections, and enables clients to determine if such a spliterator
555 * covers no elements.
556 *
557 * @return a {@code Spliterator} over the elements in this collection
558 * @since 1.8
559 */
560 @Override
561 default Spliterator<E> spliterator() {
562 return Spliterators.spliterator(this, 0);
563 }
564
565 /**
566 * Returns a sequential {@code Stream} with this collection as its source.
567 *
568 * <p>This method should be overridden when the {@link #spliterator()}
569 * method cannot return a spliterator that is {@code IMMUTABLE},
570 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
571 * for details.)
572 *
573 * @implSpec
574 * The default implementation creates a sequential {@code Stream} from the
575 * collection's {@code Spliterator}.
576 *
577 * @return a sequential {@code Stream} over the elements in this collection
578 * @since 1.8
579 */
580 default Stream<E> stream() {
581 return StreamSupport.stream(spliterator(), false);
582 }
583
584 /**
585 * Returns a possibly parallel {@code Stream} with this collection as its
586 * source. It is allowable for this method to return a sequential stream.
587 *
588 * <p>This method should be overridden when the {@link #spliterator()}
589 * method cannot return a spliterator that is {@code IMMUTABLE},
590 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
591 * for details.)
592 *
593 * @implSpec
594 * The default implementation creates a parallel {@code Stream} from the
595 * collection's {@code Spliterator}.
596 *
597 * @return a possibly parallel {@code Stream} over the elements in this
598 * collection
599 * @since 1.8
600 */
601 default Stream<E> parallelStream() {
602 return StreamSupport.stream(spliterator(), true);
603 }
604 }